1. Field of the Invention
The present invention relates to a hydrodynamic bearing device. This hydrodynamic bearing device is ideal for use in motors for information technology equipment, including a spindle motor used in a magnetic disk apparatus such as HDD and FDD, an optical disk apparatus such as CD-ROM and DVD-ROM, and a magnetic optical disk apparatus such as MD and MO, and a polygon scanner motor used in a laser printer (LBP), and is also ideal for use in small motors for electrical equipment such as an axial fan.
2. Description of the Related Art
Various types of motors, as described above, are required to rotate with high accuracy, at high speed and with low noise, and to be manufactured at low cost. A bearing for supporting a spindle of the motor is one of elements which determines the required performances of the motor. To achieve the high performance of the motor, a hydrodynamic bearing device has been studied to be used, or has been used in practical applications in recent years.
Japanese Patent Laid-Open Publication No. 2002-61641 discloses one example of the hydrodynamic bearing device. The hydrodynamic bearing device comprises a cylindrical housing with a bottom, a bearing member fixed in an inner periphery of the housing, an axial member inserted into an inner peripheral surface of the bearing member, and a radial bearing part and a thrust bearing part for rotatably supporting the axial member in a non-contact manner by the action of dynamic pressure which is generated by the relative rotation between the axial member and a bearing sleeve.
In the thrust bearing part, the dynamic pressure of oil generates pressure in a clearance of the thrust bearing part, between one end face of a flange part and the bottom surface of the housing opposed to each other, and between the other end face of the flange part and the end face of the bearing sleeve opposed to each other, in order to support the axial member in a non-contact manner in an axial direction.
In the above thrust bearing part, it is necessary to form a groove for generating the dynamic pressure (a dynamic pressure generating groove) in any face opposed to the clearance of the thrust bearing part (either end face of the flange part, the bottom surface of the housing, or the end face of the bearing sleeve). Taking a case of forming the dynamic pressure generating groove in the bottom surface of the housing, for example, the dynamic pressure generating groove may be directly formed on the inner side of the bottom surface of the housing which is integrally formed in the shape of the cylinder with a bottom. Otherwise, the dynamic pressure generating groove may be formed in the end face of a thrust plate which is in the different body from the housing. In this case, the thrust plate is attached to an end opening of the housing to form the bottom surface thereof.
Using etching or electrolytic machining makes it possible to form the dynamic pressure generating groove in the end face of the thrust plate with high precision. The etching and electrolytic machining, however, cost too much. Accordingly, it is considered to form the dynamic pressure generating groove by press working. When pressing a thin body 31 such as a thrust plate with a stamping die 32, as shown in FIG. 9a, however, a molded body 31′ tends to bend convexly on a stamping die side, as shown in FIG. 9b. 
Thus, the flatness of a molded surface becomes worse, and the depth of the groove becomes uneven. Using the non-flat thrust plate tends to cause decrease in bearing performance due to the lack of necessary dynamic pressure, and shorten its endurance life due to the frequent occurrence of contact with the axial member.